1. Field of the Invention
The present invention relates to a technique of measuring the color of a printing material containing a fluorescent brightener under a predetermined viewing illuminant.
2. Description of the Related Art
A color conversion table has conventionally been used to output a desired color from a printing apparatus. To create the color conversion table, a plurality of color patches are output to measure their colors. Then, various correction processes are performed so that a desired color can be expressed.
A general color patch measurement method is as follows. First, a spectral intensity distribution obtained by irradiating color patches with measurement light is divided by one obtained by irradiating a perfect reflecting diffuser with the measurement light. This yields an illuminant-independent spectral reflectance. Then, the tristimulus values of the color of a color patch under a given viewing illuminant are calculated as calorimetric values using the spectral reflectance, the spectrum of the viewing illuminant, and the color matching function.    [Patent Reference 1] Japanese Patent Laid-Open No. 2004-64112
However, many printing materials used to print contain a fluorescent brightener. The fluorescent brightener absorbs ultraviolet light and fluoresces in the visible range, visually enhancing brightness. A fluorescent brightener-containing printing material is readily affected by the illuminant. If measurement light used to measure color differs from an actual viewing illuminant, the emission amount of the fluorescent brightener changes, and an apparent spectral reflectance also changes. Consequently, an actual appearance differs from colorimetric values obtained by the colorimeter.
To solve this problem, there is proposed a method of measuring the color of a color patch with a spectral radiance meter under an actual viewing illuminant (patent reference 1). However, the spectral radiance meter is expensive and requires much effort to measure a color patch.